Methods of deposition of solid phases on substrates by decomposition of volatile or gaseous compounds which contain the solid phase elements are designated as chemical vapor deposition. If this deposition takes place in the open pores of a porous substrate or in the cavities of a porous structure, then it is known as chemical vapor infiltration. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) are of significance primarily with respect to the deposition and infiltration of refractory materials such as carbon, carbides, nitrides, borides, oxides etc. (see for example W. J. Lackey, in Encyclopedia of Composites, Vol. 1, edited by Stuart M. Lee, VCH Publishers, Inc., New York, 1990, pg. 319). CVI methods allow a densification of structure or, when the porous structure consists of fibers, an introduction of a matrix and, with this, the production of composite, strengthened fiber materials.
Both chemical vapor deposition as well as chemical vapor infiltration are extremely complex processes. In the chemical vapor deposition of compounds such as e.g. silicon carbide, there exists the additional problem that it is in general difficult to attain solid phase deposition in stoichiometric compositions for larger parts in the generation of a coat of even thickness.
In chemical gas phase infiltration there is another and particular problem, that the volatile or gaseous starting compounds must be transported into the depth of the pores before dissolving. If decomposition occurs on the surface of the porous structure of in the pore entrances, then the pores become clogged. The pores are then not filled, which is the whole aim of the process.
Various embodiments of methods for chemical vapor infiltration (CVI) are known.
Procedurally the simplest to perform are methods of isobaric and isothermic chemical vapor infiltration. In this method the entire process space exists at constant temperature and pressure. Here, however, only very low pressures or partial pressures of educt gases can be used, when necessary with addition of inert or dilution gases, so that extremely long infiltration times are required.
In order to shorten the infiltration times it is proposed according to WO 95/16803 that for the chemical vapor infiltration of silicon carbide using methyltrichlorosilane (MTS) as educt gas, the educt gas should be preheated to temperatures well above the decomposition temperature of MTS i.e. to 960.degree. C. to 1050.degree. C. while at the same time setting pressures up to 25 kPa and to remove silicate components from the gas phase at the outlet of the reaction zone. Preheating the MTS to such high temperatures leads to a high rate of deposition of the substances added with the gas, which in turn achieves a high production speed but at the same time leads to uneven deposits, particularly on the surface, and therefore to minimal extents of pore filling.
Optimal or maximal pore filling is therefore only possible at extremely slow deposition or infiltration rates (e.g. W. V. Kotlensky, in Chemistry and Physics of Carbon, Vol. 9, edited by P. L. Walker and P. A. Thrower, Marcel Dekker, New York, 1973, pg. 173).
In order to successfully realize infiltration, low pressures, and particularly low partial pressures are recommended. The pressures realized under the conditions of industrially applied chemical vapor infiltration lie at least one to two orders of magnitude below normal pressure. Starting compounds are partially mixed with inert gases so that their partial pressure, and with it the deposition rate can be further lowered. Due to the low partial pressures, extremely long infiltration times of up to several weeks are required.
Since the isobaric and isothermic methods failed in achieving rapid production and high degrees of pore filling, the development of new methods was attempted, known as pressure gradient, temperature gradient and pressure switching methods. Such methods are for example known from Nyan-Hwa Tai and Tsu-Wei Chou, Journal of American Ceramic Society 73, 1489 (1990).
In the vacuum pressure pulsation method, the process pressure is continually varied to support the diffusion. The disadvantage of this method lies in the cost of the apparatus as well as in the filtration times, which are still very long.
Another well-known method is the temperature gradient method (e.g. U.S. Pat. Nos. 5,411,763, 5,348,774). In this method heat is removed from the side of the porous substrate facing the process gas stream by suitable measures, for example by cooling by the stream. The side of the porous substrate opposite to the gas stream is adjacent to a heating element. It is in this way that a temperature gradient crucial to the method is established normal to the surface of the substrate. The surface temperature on the cold side is adjusted with the gas stream such that no, or at least very little deposition takes place. It is in this way that narrowing of the pores in this region is avoided. The disadvantage of this method is the very high gas throughput necessary for cooling. The low yield of deposited material entails long production times. Much equipment is needed for the heating.
In a further known embodiment of CVI methods (DE 41 42 261) the gas is streamed through the porous substrate on the basis of forced convection whereby a pressure gradient is established. The infiltration time can be kept relatively short. After a certain level of pore filling however, the streaming through of the porous structure becomes more difficult.
From U.S. Pat. No. 4,580,524, a CVI method is known whereby temperature and pressure gradient techniques are combined with one another. In this way relatively short production times can be achieved. The disadvantage of such a method is the complicated reactor construction.
The task which provided the basis for the invention was to create a CVI method by which a high level of pore filling during a pre-set production time could be achieved, or alternatively, a shorter production time achieved for pre-set pore filling levels.